This application claims priority from European Patent Application 02080456.3, filed Dec. 23, 2002 which is herein incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates generally to lithographic apparatus and more particularly to lithographic apparatus including spatially modulated illumination nodes.
2. Description of the Prior Art
The term “patterning device” or “patterning structure” as here employed should be broadly interpreted as referring to devices that can be used to endow an incoming beam of radiation with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning device include:                A mask. The concept of a mask is well known in lithography, and it includes mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. Placement of such a mask in a radiation beam causes selective transmission (in the case of a transmissive mask) or reflection (in the case of a reflective mask) of the radiation impinging on the mask, according to the pattern on the mask. In the case of a mask, the support structure will generally be a mask table, which ensures that the mask can be held at a desired position in the incoming radiation beam, and that it can be moved relative to the beam if so desired;        A Programmable Mirror Array. One example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. The basic principle behind such an apparatus is that (for example) addressed areas of the reflective surface reflect incident light as diffracted light, whereas unaddressed areas reflect incident light as undiffracted light. Using an appropriate filter, said undiffracted light can be filtered out of the reflected beam, leaving only the diffracted light behind; in this manner, the beam becomes patterned according to the addressing pattern of the matrix-adressable surface. An alternative embodiment of a programmable mirror array employs a matrix arrangement of tiny mirrors, each of which can be individually tilted about an axis by applying a suitable localized electric field, or by employing piezoelectric actuation means. Once again, the mirrors are matrix-addressable, such that addressed mirrors will reflect an incoming radiation beam in a different direction to unaddressed mirrors; in this manner, the reflected beam is patterned according to the addressing pattern of the matrix-adressable mirrors. The required matrix addressing can be performed using suitable electronic means. In both of the situations described hereabove, the patterning device can comprise one or more programmable mirror arrays. More information on mirror arrays as here referred to can be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096, which are incorporated herein by reference. In the case of a programmable mirror array, said support structure may be embodied as a frame or table, for example, which may be fixed or movable as required; and        A Programmable LCD Array. An example of such a construction is given in U.S. Pat. No. 5,229,872, which is incorporated herein by reference. As above, the support structure in this case may be embodied as a frame or table, for example, which may be fixed or movable as required.        
For purposes of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning device as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning device may generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The illumination system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables while one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.
The illumination system has to ensure the realization of a desired intensity distribution at the substrate as a function of position across the beam and as a function of angle of incidence of different rays in the beam. The desired position dependence, excluding mask generated patterns, generally has to be uniform with constant intensity as a function of position on the substrate, and the desired angle dependence has to peak at certain angles. The desired angle dependence may depend on the nature of the pattern on the mask. Since different patterns need to be imaged, it has to be possible to change the angular dependency of the illumination of the mask.
European Patent application EP 1109067, which is incorporated herein by reference, describes examples of such an illumination system. Herein, the illumination system has a laser source, followed by an optical element such as a Diffractive Optical Element (DOE) and a lens. The lens is followed by an optical sub-system that passes the beam to the mask. The optical sub-system uses a plane after the DOE and the lens as a pupil plane for illuminating the mask. The pupil plane is in the focus plane of the optical sub-system, so that the spatial intensity distribution of the beam in the pupil plane determines the angular intensity distribution of the beam at the mask and the substrate (“angle” in this context may refer to the angle relative to the main direction of the beam, as well as to an angle rotated around that main direction). For the sake of completeness, it may be noted that an illumination system may further comprise a reflective integrator, such as for example a quartz rod, which will cause the beam to experience a number of reflections, so that in fact the angle dependence of the intensity distribution of the beam at the mask is determined by the overlap of a number of mirrored copies of an initial intensity distribution before the reflective integrator.
EP 1109067 uses the DOE to control the spatial intensity distribution of the beam in the pupil plane, and thereby to control the angular intensity distribution at the substrate. It is described for example, how the function of an axicon (providing zero intensity in a circle in the centre of the pupil plane) can be combined with a DOE. Basically the DOE comprises an array of microlenses, each defining a region transverse to the direction of the beam, through which radiation from the beam is passed. Conventionally, the region of each microlens has a circular or hexagonal shape, which would lead to a circle or hexagonal shaped intensity distribution in the pupil plane. However, by using different shapes, different distributions can be realized. For example, by using pie shaped microlenses, which pass radiation only through a part of a circle, a pie shaped intensity distribution can be realized in the pupil plane. Similarly, a dipole shaped region, with lobes that pass radiation emanating in mutually opposite directions from the centre of the lens, can be realized in the pupil plane. By using an array of identical microlenses across the beam, the inhomogeneity of the beam does not significantly affect the intensity distribution in the pupil plane.
Dependent on the particular integrated circuit topology under process, or even the particular process step, different position dependencies of the intensity distribution in the pupil plane may be needed. For this purpose EP 1109067 provides a DOE exchange unit, so that a DOE for creating a desired intensity distribution can be introduced into the beam as needed.
Since the DOE plays a crucial role in realizing the desired intensity distribution in the pupil plane, it is usually necessary to design DOE's specifically for different possible desired intensity distributions and to change the DOE, dependent on the desired distribution. However, if a specific DOE must be designed and made for each possible desired intensity distribution, a considerable delay for designing and manufacturing the DOE results each time a new intensity distribution is needed. Also a very large number of DOE's is needed in this way.
EP 1109067 also describes various ways to place a number of different DOE's in parallel in the incoming beam. Each DOE redirects a part of the beam cross-section, so that the intensity at the pupil plane is a sum of contributions from different DOE's. New intensity distributions can be generated by using different combinations of DOE's without manufacturing new DOE's.
Of course this requires a homogeneous distribution of the incoming beam over the various DOE's. Thus, one of the main advantages of using DOE's is sacrificed to a certain extent: the fact that an array of microlenses with identical effect is used in parallel, so that inhomogeniety of the beam does not matter. To alleviate this inhomogeneity a large number of parallel DOE's is required, assembled at random over the beam area. This makes assembly and re-use complicated.
Furthermore EP 1109067 describes serial placement of different DOE's in the path of the beam. For example, one DOE generates a ring-shaped pupil (which is normally done by an axicon) while another (serially placed) DOE generates a filled, circular pupil. The effect of serial placement is to convolute the position dependencies imparted by the various DOE's to the intensity distribution in the pupil plane. Convolution of a first and second intensity pattern has the effect of spreading each point of the first intensity pattern with a distribution determined by the second pattern. Thus convolution can be used for example to widen a ring in a pattern where the intensity is limited to a ring shaped region in the pupil plane. However, convolution generally does not provide refined control over the intensity distribution, in particular when a number of DOE's are placed in series. Accordingly, EP 1109067 uses the convolution of different DOE's in serial placement for specific purposes, like the above mentioned broadening of the ringwidth (instead of using zoom-optics). Another example is that one DOE generates a circular pupil which due to different optical losses in the x- and y-direction gets non-circular (e.g. elliptical) at reticle level. This might lead to different widths of horizontal lines as compared to vertical lines on the wafer. This ellipticity can be compensated by serially inserting a second DOE which radiates preferably in one direction, the convolution of which with the first DOE gives the wanted circular pupil at reticle level.
However, for other types of composition convolution is less suitable. Thus specific new intensity distributions in the pupil plane, for example, generally still require parallel arrangement of DOE's or a lengthy and costly manufacture of new DOE's.